What is Radio Astronomy?

Fundamental to the SKA, many people ask “What is radio astronomy”. People are used to the stunning visual images taken by telescopes like the Hubble or the great telescopes in Hawaii or Chile, but maybe only have heard of radio astronomy through movies like “Contact”.

Since the 1930s, when the first radio signals from space were detected by Karl Jansky, astronomers have used radio telescopes to explore the Universe by detecting radio waves emitted by a wide range of objects. Our Sun, the nearest star to Earth is a powerful radio emission source, mainly due to its proximity to our planet, but some radio sources, which are millions, even billions of light years away, are truly colossal in terms of their radio output.

Radio astronomy progressed through the middle of the last century, with many great discoveries made in radio frequencies such as the discovery of pulsars by Dame Jocelyn Bell Burnell, a postgraduate student working at the University of Cambridge whilst a student under Anthony Hewish, who went on to share the Nobel Prize for Physics with Martin Ryle, another notable radio astronomer in part for this discovery.

So how do radio telescopes work?

Karl Jansky: Father of Radio Astronomy

Radio telescopes provide alternative views to optical telescopes, they can detect invisible gas, and can reveal areas of space that may be obscured with cosmic dust.

Unlike optical telescopes, which can be hampered by cloud or poor weather conditions on Earth, radio telescopes, working with signals at a longer wavelength, can be used even in cloudy skies. The longer wavelength of radio emissions means that the radio telescopes used to detect them do not have to be as perfectly shaped as their optical counterparts, (but still need to be accurate to around 1 mm in terms of accuracy of the dish shape) but, to obtain the same level of detail and resolution as their optical cousins, radio telescopes have to be much larger or have a much larger collecting area, as light is a much shorter wavelength. The largest radio telescope in the world as a single dish, is the Arecibo telescope, which featured in the movie “Contact”, and is located in a natural hollow in Puerto Rico, South America.

Even the headquarters of the SKA Organisation, at Jodrell Bank in the United Kingdom, is steeped in history. Home to one of the largest fully steerable radio telescopes in existence, the 76m Lovell telescope, one which not only worked on detection radio emissions from deep space, but that also played a pivotal role in the great space race of the 1960s between Russia and the United States.

The telescope at Jodrell Bank, named after Sir Bernard Lovell, a pioneer in radio astronomy and the founding father of the Jodrell bank site, tracked both the first spacecraft “Sputnik” and also many of the U.S and Soviet-era spacecraft on their travels from the Earth to the Moon and beyond.

If we could look at the sky in radio wavelengths with our own eyes, what would we “see”?

Impression of what the Night Sky Looks like in Radio Wavelengths : (c) National Radio Astronomy Observatory / Associated Universities, Inc. / National Science Foundation

Even with the 76m aperture size of the Lovell and over 300 metres for the Arecibo telescopes, the resolution of optical telescopes is still vastly superior. To match the resolution you need a radio telescope kilometres across, this for a single dish is clearly not possible or practical.

With a single dish the size of Arecibo, the limitation is one of physical size in the structure. This vast telescope, suspended in a natural bowl has limited movement across the sky, and limited ability to point at certain regions.

Also with a single telescope approach, you are pointing your telescope at a single source only, the SKA aims to circumvent this by sheer scale of numbers, and also using clever technology.

But the issue still remains as to resolution vs optical telescopes, so to get around this limitation in size, radio astronomers have been able to utilise a technique known as interferometry.

What is Interferometry?

How a basic interferometer in radio telescopes works

Radio telescopes (and more recently optical ones) can be used individually or they can be linked together to create a telescope array known as an interferometer. The SKA will be one of the world’s largest interferometers and by far the most sensitive, spanning thousands of kilometres. The main bulk of the telescopes will be located at core regions in South Africa and Australia. If all the individual telescopes that made up the SKA were combined into one telescope dish, it would be equivalent to one that is a kilometre square in size, hence the name.

The resolution of an interferometer depends not on the diameter of individual radio telescopes, but on the maximum separation between them.

Moving them further apart increases the angular resolution (the telescope ability to resolve smaller objects in the sky). In an interferometer, the signals from all of the telescopes are then brought together and processed by a correlator, which combines the signals to effectively simulate that from a single much larger telescope. With so many telescopes in this SKA, not only the angular resolution, but also the telescope sensitivity will be like nothing ever seen before.

When complete, the SKA will surpass the resolution of optical instruments like the Hubble Space Telescope by some factor.

The Radio Spectrum – Image credit: CEPL

Detecting the invisible sky

Radio signals pass straight through the clouds and can be detected by these large antenna/dishes or receptors as the radio telescopes are known. The radio frequency range covers a vast swathe of the electromagnetic spectrum between around 30 MHz and 40 GHz, which is equivalent to, wavelengths from 10 m down to 7 mm. The SKA will observe at a frequency range from 50 MHz to 20 GHz which is equivalent to wavelengths of 4 m to 3 cm.

The Basic physics of how radio emission works.

Principle in the detection of radio signals from space is the Hydrogen atom.

Radio emission. Image: SKA Organisation

The hydrogen atom comprises a proton and an electron. While not strictly little spheres, both the electron and proton do have a property known as ‘spin’. The spins of the two particles can be aligned or anti-aligned. If spins of the electron and proton in a hydrogen atom are aligned, the atom has slightly more energy than if the spins are anti-aligned.

A hydrogen atom can make a transition from the aligned state to the anti-aligned state. In doing so, it emits radio energy at a wavelength of 21 cm or a frequency of 1420 MHz.

Conversely, in order for the atom to make the transition from anti-aligned to aligned, the atom has to be exposed to 21 cm wavelength radiation, from which it can absorb radio energy.

This 21cm line as it is known in radio astronomy is a fundamental to radio astronomy and mapping it will be key to the operation of the SKA

Comparing views

The sky seen at optical wavelengths. Credit: Axel Mellinger/NASA SkyVie

In the image above, we can see a typical optical view of the sky, with our galaxy, the Milky Way dominating the centre of the image. In the next image, taken with a radio telescope, we see the fundamental differences, (yet with some overlap) with the optical image above. This is an image of the 21cm line map of the entire Sky. It is maps like this which the SKA will be able to produce thousands of times faster and with a higher resolution than any other sky survey has been able to before

The sky seen at the radio frequency of neutral hydrogen emission (21cm/1 420 MHz). Credit: J.Dickey/NASA SkyView

With the image below, we see a range of frequencies with the different intensity of radio emission colour coded, with red being the strongest and blue being the weakest. As you can see, if you compare this to the optical image above, our own galaxy the Milky Way is a very strong emitter of radio waves. Once fully operational in the early 2020s the SKA will be the most powerful, sensitive and largest radio telescope ever constructed.